1. Field of the Invention
This invention relates to the production of terephthalic acid. More particularly it relates to an enhanced process and system for the production of said terephthalic acid.
2. Description of the Prior Art
In a typical air or enriched air based process for producing terephthalic acid, liquid p-xylene is fed to a stirred tank reactor, with a monobasic aliphatic acid, typically acetic acid being used as a solvent. The ratio of solvent to reactant is typically one to ten weights of solvent per volume of reactant (1:1 to 10:1). The reaction is catalyzed with a heavy metal or mixture of heavy metals, most commonly cobalt and manganese in the form of acetate salts. In addition, bromine, in the form of bromic acid, is commonly used as an initiator. The reactor is maintained at an operating temperature of between 170.degree. C. and 225.degree. C. The operating pressure is generally between 100 and 300 psig. Compressed air or enriched air, typically having between 21% and 28% oxygen, is sparged into the bottom of the reactor. Oxygen from the air is dissolved into the liquid phase and reacts with the p-xylene to produce the desired terephthalic acid product. Intermediate oxidation products and by-products are also formed in quantities that depend on the reaction conditions employed. At a residence time of one hour, the conversion of p-xylene is typically about 99%, with the yield to desired terephthalic product being greater than 96%.
The most important intermediate oxidation product in the production of terephthalic acid (TPA) is 4-carboxybenzaldehyde (4-CBA), which is one oxidation step removed from terephthalic acid. The presence of 4-CBA in the TPA produce is undesirable. It acts as a chain terminator in subsequent polymerization reactions which convert TPA to its most important end products, i.e., polyester fibers and polyethylene terephthalate resins. For a given residence time, the conversion of 4-CBA to TPA has been observed to increase with temperature. Hence, the concentration of 4-CBA in the TPA product decreases with increased operating temperature, so that TPA product quality increased at higher operating temperatures.
Raw material losses to undesirable byproducts, on the other hand, also increase with temperature. The acidic acid solvent and, to a lesser extent, p-xylene, react to produce carbon dioxide, carbon monoxide, methyl bromide and methyl acetate, all of which are environmentally sensitive materials. Since a high reaction temperature must be maintained to make product terephthalic acid that meets applicable quality standards, the loss of acetic acid and the commensurate production of byproduct gases is usually a significant factor in the economics of the overall operation.
In such known operations, feed air must be compressed to a pressure somewhat above the reactor operating pressure before it is blown into the reactor through a pipe or other submerged sparger. The air bubbles are dispersed in the reactor and are circulated through the body of liquid reactant and solvent by an agitator device. The oxygen concentration in the air bubbles decreases as the oxygen dissolves and reacts with the p-xylene. The residual air bubbles disengage from the liquid phase and collect in a gas space at the top of the reactor to form a continuous gas phase. This waste gas must be vented in order to provide space for fresh air feed, while maintaining adequate gas hold-up in the reactor to promote the desired oxygen transfer from the air to the liquid phase.
To avoid the possibility of fire or explosion, the oxygen concentration in the gas space at the top of the reactor must be maintained below the flammable limit. For practical operating purposes, the oxygen concentration must be maintained at less than 8-9% by volume. More typically, the oxygen concentration in the gas space is maintained below 5% by volume to provide a safe margin below the flammable limit. Thus, in a well stirred tank reactor, the average concentration of oxygen in the circulating air bubbles must be below 5% in order to insure that the average concentration of oxygen in the gas that collects in the headspace of the reactor is nonflammable.
The oxygen concentration in the gas space is a function of the rate at which air or enriched air is fed into the reactor and the rate of consumption of oxygen from the air by reaction with p-xylene. The rate of reaction and, therefore, the TPA production rate per unit of reactor volume, increases with temperature, pressure, oxygen concentration in the gas phase, p-xylene concentration, promoter concentration and catalyst concentration. Since the concentration of dissolved oxygen in the liquid phase, and, hence, the reaction rate of oxygen, is proportional to the oxygen concentration in the gas phase, for a given set of reaction conditions, the 5% oxygen restriction in the headspace effectively limits the oxygen reaction rate.
Clearly, air or said enriched air, typically 21% to 28% oxygen, based TPA plant design requires optimization of temperature, pressure, catalyst loading, air feed rate, reactor volume, and vent gas treatment equipment. For example, increasing temperature increases productivity per unit reactor volume and improves product purity, but it also leads to yield and solvent losses, and byproduct gas formation due to over oxidation.
It has more recently been proposed to use oxygen, or nearly pure oxygen, as an oxidant in the TPA production process. Such an oxygen based process for TPA production would typically be carried out in a conventional reaction vessel employing direct contact cooling devices, for example cooling coils, to remove the heat of reaction from the vessel and to maintain the desired operating temperature. Such oxygen based TPA production, carried out in a reactor adapted to obviate the potential for fire or explosion, would desirably be carried out under TPA operating conditions serving to minimize the amount of undesired by-products present in the terephthalic acid product and the amount of vent gases to be treated as part of the overall production operation.
In the commercial practice of otherwise desirable oxygen based terephthalic acid production operations, as in conventional air based TPA operations, a disadvantageous feature is occasioned by the fact that the desired TPA product of p-xylene oxidation is a solid product. As a result, the reaction mixture is supersaturated with TPA, and the solid TPA reaction product readily precipitates onto any cooled surface. Consequently, the cooling coils typically used for the direct cooling of the reactor to the desired operating temperature rapidly become coated with the solid TPA product and lose much of their heat transfer capability. This results in premature shut-down of the TPA production operation for maintenance purposes, adding considerably to the overall costs associated with TPA production.
As the production of TPA is a highly significant commercial operation, there is a genuine need in the art for an improved TPA process and system. In particular, the loss of important TPA production time due to the above-indicated heat transfer problems must be overcome to enhance the overall efficiency of the TPA process and system. As oxygen based TPA production operations are particularly desirable, such improvement in the art needs to be applicable to oxygen based TPA operations so as to enhance the feasibility of their use in preference to conventional air based TPA production operations.
It is an object of the invention, therefore, to provide an improved process and system for the production of terephthalic acid.
It is another object of the invention to provide an improved oxygen based TPA production process and system.
It is a further object of the invention to provide a process and system for TPA production obviating the need for premature shut-down because of the loss of heat transfer capability and cooling effectiveness upon precipitation of the TPA product.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended Claims.